An important feature of modern electronic systems is the ability to adapt to evolving technologies and growth requirements. Electronic systems which are unable to adapt run the risk of becoming obsolete. Thus, the electronic designer should anticipate growth and provide a flexible architecture allowing for future growth.
An important component of every electronic system is the power supply. If the power supply is not expandable then system growth is either impossible or includes the task of replacing the original power supply with one capable of handling larger power demands. Two standard methods of furnishing a power supply that is expandable are: providing a power supply that has large unused capacity, and providing a power supply which can be inexpensively enlarged to meet future requirements. The latter of the two possibilities is more desirable because the system purchaser does not pay for the extra capacity until it is used.
One approach to the enlargeable power supply problem has been to provide a number of power supplies with paralleled outputs, as in U.S. Pat. Nos. 4,177,389 issued Dec. 4, 1979, and 4,174,534 issued Nov. 13, 1979. Both of these patents involve the paralleling of two substantially identical power supply units to achieve current levels exceeding the capability of a single unit. Substantially similar units provide the system purchaser with the benefits of interchangeability, which tend to reduce cost for expansion units, as well as, increase availability for maintenance and replacement units. However, when two or more interchangeable units which are each capable of independent output control and regulation are paralleled, a problem occurs with regard to the control of the units for sharing the output current load.
In U.S. Pat. No. 4,177,389, the interconnection and control allocation is more complex than necessary for just current sharing units. This is because the patent also encompasses an "on-line" redundancy in which either power supply can assume the voltage and current requirements of the entire system in the event of a fault in one of the paralleled units. In the above identified patent, each power supply has a monitoring circuit which will switch over control to itself in the event of a fault in the paralleled, redundant supply. Recognizing the current sharing problem, the U.S. Pat. No. 4,177,389 provides for the comparison of the average D.C. current of the voltage regulation (i.e., master) unit with the average D.C. current into the current regulated (i.e., slave) unit, and utilizes PWM (pulse-width-modulator) controlled circuitry to minimize the difference of these two currents. The apparatus of this patent addresses the danger of positive feedback between the master and slave units with a current control circuit which is considerably delayed by D.C. filtering, and PWM control circuits which are deliberately operated out of synchronization with each other. Such a design does not truly share the load current. Since the frequency and duration of current from each unit is supplied asynchronously, at any one time, instead of a 50%-50% load current sharing, there can be a 60%-40% or even more unbalanced division of the load current during steady state load conditions. Further, non-steady state load conditions, because of the delay times built into the control circuitry, will lead to even greater instantaneous unbalanced load current division conditions. Thus, in applications where "on-line" redundancy is not required, it is desirable from cost and complexity aspects to provide a simpler circuit with more equal current sharing as the solution to the problem of controlling paralleled power supply units.
A less complex design for solving the current sharing problem when paralleling two power supplies is the non-redundant master/slave arrangement in which one interchangeable unit is assigned the duties of controlling its own outputs as well as the outputs of a slave unit. For interchangeable units, this requires the inclusion of control outputs allowing each the capability to act as a master, as well as, control inputs allowing each unit the capability of acting as a slave. Further, when the control signals of the master unit are connected to the slave unit, provision also must be made that the slave unit's own control signals are either disconnected or are overridden by the master control signals. An example of a known master/slave power supply is shown in FIG. 3 of U.S. Pat. No. 4,174,534, in which unit 65b is the master and unit 65a is the slave. Each unit has a double-pole, single-throw switch 66, 67 which is open if the unit is to be the slave which is controlled by the master unit, or closed if the unit is to be the master unit which provides the control.
The circuit of this non-redundant master/slave power supply is still unnecessarily complex in that many components can be eliminated by more effectively utilizing every component. Moreover, the master/slave switches present a problem because both switches could be inadvertently closed, in which case the two control circuits would both be constantly trying to control the output, or both switches could be inadvertently opened, in which case the system could be operating in an open loop condition with possible catastrophic consequences. Thus, it is desirable for a master/slave power supply circuit to be only as complex as necessary to perform its function, and also to have control loops that cannot be inadvertently, incorrectly connected.